external hard disk files

[cybogs]

i cannot access some of my datas in some of the files in my external hard disk anymore. there are some which i can access but the next time i would try the same data it could not be accessed anymore. a warning would remind me to check if the disk is full, or write-protected. the disk is neither full nor write-protected. zone alarm also did not detect a virus. although ZA did detect 7 spywares, all were treated and deleted. if ever all the datas and files are corrupted how can i recover them? what software should i use? is norton save and restore good enough? please send me other options.